HunCraft Genocide is the most detailed unofficial expansion pack for StarCraft: Brood War. It has 3 new units, new heroes and special buildings in the campaign, 3 new campaign episodes (26 missions and a hidden mission), more than 40 new multiplayer maps and some small changes. Though the most StarCraft expansion packs can be used in use map settings, this game has own exe file, menus and new map file format (hcx). It does not change anything to your original StarCraft installation, but needed to be in the same directory. There is a bugfix patch for it which make some balances in the game.
Single player can be described with mass battles and especially difficulty.
The story continues after Brood War, when admiral DuGalle has committed suicide and Kerrigan has regained control of the Zerg forces. According to Brood War epilogue, no UED ships returned to Earth, but some UED forces remained on Char. The game starts with exactly the same prologue which Brood War has as an epilogue.